【作者】 郭海强；

【导师】 赵斌；

【作者基本信息】 复旦大学 ， 生态学， 2010， 博士

【摘要】 本文以上海崇明东滩河口湿地为例,通过地面监测、遥感分析和模型模拟等途径,探讨了河口湿地碳通量的变化及其影响因子,并特别分析了潮汐作用和植物入侵对河口湿地碳通量的影响以及遥感模型在湿地生态系统总初级生产力估算中的应用。本研究的主要结论如下：(1)在2005-2007年期间,崇明东滩河口湿地表现为较强的碳汇,其固碳强度为562-834 gCm-2 yr-1,远高于目前已报道的其他类型湿地；其碳循环各分量(生态系统总初级生产力,生态系统呼吸和二氧化碳净交换量)表现出明显的季节和年际动态。在2005年和2007年,分别位于较高潮滩和较低潮滩的两个站点(下面简称为高滩站和低滩站)二氧化碳净交换量(下面简称为净碳交换量)的季节动态具有显著性差异,其年累计净碳交换量之差高达100 gCm-2 yr-1以上。在大多情况下,两站点的净碳交换量对气象因子的响应是一致的,但在极端状况下则表现出明显差异,这说明对于具有环境梯度的生态系统而言,净碳交换量本身及其对环境因子的响应均存在一定的异质性。显然,这些因素对全球变化研究中的尺度上推增添了较多的不确定性。(2)崇明东滩河口湿地会受到潮汐作用的影响,其周期为15天,本研究观测到净碳交换量也表现出类似的周期性变化,其周期为10-20天。低滩站对潮汐作用的影响表现得更为敏感。潮汐作用通过水淹直接对土壤呼吸产生抑制来影响净碳交换量,其影响也呈现出季节性变化。白天的净碳交换量是植物光合作用与土壤呼吸作用叠加的结果,因此其对潮汐作用的响应则更为复杂,同时受植被物候与潮高的影响。考虑到潮汐作用期间潮水会将有机质转移至邻近海域,这显然将影响河口湿地碳源／汇属性的评估,因此,进一步的研究需要考虑横向碳通量。(3)根据遥感图像分析,东滩湿地的入侵植物互花米草在2005-2006年间扩张迅速,且当其盖度在20-40%间时,扩张最快。在各站点,互花米草在近海的区域比相应近岸的区域扩张更快。在研究期间,生态系统总初级生产力、生态系统呼吸和净生态系统生产力(数值上等同净碳交换量)都随互花米草盖度的增加而增加。同时,互花米草的入侵也改变了碳通量对环境因子的响应。高光照条件下的光抑制得到一定程度的缓解；生态系统呼吸对温度的响应也发生一些变化,在相对低温区域(<26℃),生态系统呼吸对温度的敏感性增加,而在相对高温区域(>26℃),对温度的敏感性则降低。(4)基于涡度协方差技术的通量监测塔只能获得几百米或几千米范围内的净碳交换量,因此其结果很难直接上推到较大的区域尺度或全球尺度。VPM模型通过耦合通量塔监测数据和遥感数据来估算生态系统总初级生产力,从而能在尺度上推中扮演重要角色。最大光能利用效率在很大程度上决定了VPM模型能否很好地拟合通量塔估算的生态系统总初级生产力。本研究发现,在VPM模型应用中,如果估算最大光能利用效率的时间窗口太长(5-10月),生态系统总初级生产力会被低估,而采用较短时间窗口估算的最大光能利用效率,可增加VPM模型的拟合度。从近海区到近岸区,VPM模型拟合的效果递增,这表明潮汐作用及水文条件也会影响到VPM模型的拟合效果。更多还原

【Abstract】 With the help of eddy covariance and remote sensing technique, we investigated the carbon dynamic patterns and the associated driving factors and also tried to simulate gross primary productivity of an estuarine wetland located on Chongming Island of Shanghai, China. Meanwhile, we examined how plant invasion and tidal activity affect wetland carbon fluxes. The main findings are summarized as follows:(1) During three years observation (2005-2007), the wetland behaved as strong C-CO2 sink, with the strength of 562-834 g C m-2 yr-1. Seasonal patterns of net ecosystem exchange of CO2 (NEE) showed significant differences between high and low elevation sites in 2005 and 2007, and the cumulative NEE at high elevation site was higher than that at low elevation site by a magnitude of 100 g C m-2 yr-1. The carbon fluxes showed similar responses to meteorological variables at two sites, but differed largely in extreme conditions. The decreased tidal activity along the altitudinal gradient, combined with the different plant speceis composition, mainly accounted for the different behaviors of carbon fluxes. This study identified the heterogenity of carbon fluxes and their response to meteorological variables inside ecosystems with environmental gradients, which brought great uncertainty in whole ecosystem estimation and upscaling.(2) One year of continuous data from high and low elevation sites were analyzed to evaluate the tidal effect on carbon flux. The measured wavelet spectra and cospectra of NEE and other environmental factors demonstrated that the dynamics of NEE at both sites exhibit a tidal-driven pattern with obvious characteristics at scales between 10 and 20 days (256-512-h). Environmental factors exerted major controls on the carbon balance at finer temporal scales. NEE was more sensitive to tides at the low elevation site than at the high elevation site. Overal the mean nighttime NEE during spring tides was lower than that during neap tides, indicating suppressed ecosystem respiration under inundation. Larger differences were observed at the low elevation site due to the longer inundation. In contrast, daytime NEE was more variable since plant reacted differently in different growth period and under different tidal elevations. Whilst tides would also transport organic matter to nearby estuary and hence may incur carbon emission in the receiving ecosystems. Thus, further study on lateral carbon transport is required to investigate the tidal effect on the carbon sink/source role of the wetland.(3) From 2005 to 2006, the percent of cordgrass (Spartina alterniflora) increased by 10-30% on the wetland, and largest increases were observed when the previous percent fell in the range of 20-40%. For each site, the fetch that faced the sea experienced larger increases than that faced the dike. Gross primary production (GPP), ecosystem respiration (RE) and net ecosystem productivity increased with rising percent of S. alterniflora. The increased carbon sequestration rates in C3 species community invaded by C4 species S. alterniflora showed potential in alleviating climate warming. The invasion of S. alterniflora also alterd the responses of carbon flux to environmental factors. With greater percent of S. alterniflora, the light inhibition under strong insolation was relieved; the temperature sensitivity increased when temperature was below 26℃, while increased when temperature rose above 26℃.(4) Vegetation photosynthesis model (VPM), which combines the measurements of eddy flux tower and satellited-derived data, shows robust in simulating ecosystem productivity and thus acts as a strong candidate for upscaling work. We tried two methods to calculate maximum light use efficiency (e0), which in large part determined the performance of VPM. e0 derived from long-term (almost whole growing season) dataset would lead to an underestimate of GPP. In comparison, use of short-term eo greatly enhanced the simulation. This was mainly due to the summer passive condition and scattered distribution of data points for estimating eo. Meanwhile, uptake of CO2 by canopy was more like instantaneous response. Best simulation was observed at high elevation site. We proposed that the tidal activity affected the reflectance and associated vegetation indices (VIs), which was partly reflected in the dynamics of VI along the altitudinal gradient. Thus, a refinement of VI is required for ecosystems that experience flooding. Some other reasons, like bias in the partitioning of NEE into GPP and RE, different weather conditions on days that satellite does not pass by was also discussed.更多还原